Available online at www.sciencedirect.com
Available online at www.sciencedirect.com
ScienceDirect
ScienceDirect
Procedia
Engineering
00 (2017) 000–000
Available
online
at www.sciencedirect.com
Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
Procedia Engineering 202 (2017) 305–311
4th International Colloquium "Transformer Research and Asset Management”
4th International Colloquium "Transformer Research and Asset Management”
Transferred Voltages due to Single Phase Earth Fault on Power
Transferred Voltages due to Single Phase Earth Fault on Power
Transformers
Transformers
Kosjenka Capuder Bikića,a,*, Mislav Gazdovićaa, Franjo Kelemenaa, Aleksandar Lojpuraa
Kosjenka Capuder Bikić *, Mislav Gazdović , Franjo Kelemen , Aleksandar Lojpur
a
a
Končar Power Transformers, Josipa Mokrovića 12, Zagreb, Croatia
Končar Power Transformers, Josipa Mokrovića 12, Zagreb, Croatia
Abstract
Abstract
In the event of an earth fault in one of the two systems of a three-winding transformer, an increase in the starIn voltage
the event
of aninearth
fault in one
of the
twovoltage
systems
a three-winding
transformer,
an increase
the starpoint
occurs
the disturbed
system.
This
is of
transferred
to the undisturbed
systems
due toincapacitive
point
voltage
occurs
in
the
disturbed
system.
This
voltage
is
transferred
to
the
undisturbed
systems
due
to
capacitive
coupling, to affected earth faulty system and to earth. As a result, three phase voltages to earth are different in
coupling, toand
affected
earth faulty
systemto and
earth. As aservice
result,status.
three phase voltages to earth are different in
magnitude
phase position
compared
theirtoundisturbed
magnitude
phase
compared
to theirisundisturbed
service the
status.
In this and
paper
the position
mesh analysis
method
used to calculate
neutral point shift of the faulted winding
In thistopaper
meshofanalysis
method
is used
calculate winding
the neutral
pointwhich
shift isofnot
theearth
faulted
winding
necessary
obtainthe
values
transferred
voltages
to thetounaffected
(the one
faulted).
The
necessary
obtain values
of transferred
thewinding
unaffected
one which
is not earth
faulted).
The
results
aretoanalyzed
to investigate
the voltages
influencetoof
andwinding
network(the
neutral
grounding
(insulated,
solidly
results aregrounded
analyzedover
to investigate
influence
grounded,
impedance)the
on the
results.of winding and network neutral grounding (insulated, solidly
grounded, grounded over impedance) on the results.
©
© 2017
2017 The
The Authors.
Authors. Published
Published by
by Elsevier
Elsevier Ltd.
Ltd.
©
2017 The Authors.
Published by
Ltd. committee
Peer-review
under
of
the
Peer-review
under responsibility
responsibility
of Elsevier
the organizing
organizing
committee of
of ICTRAM
ICTRAM 2017.
2017.
Peer-review under responsibility of the organizing committee of ICTRAM 2017.
Keywords: Power transformer, transient voltages, overvoltages, ground fault, neutral phase shift
Keywords: Power transformer, transient voltages, overvoltages, ground fault, neutral phase shift
1. Introduction
1. Introduction
The most frequent causes of temporary over voltages are faults, load rejection, resonance and ferroresonance.
most frequent
temporary
over
voltages
areon
faults,
load rejection,
ferroresonance.
TheThe
magnitude
of over causes
voltagesofdue
to ground
faults
depends
the method
of systemresonance
groundingand
(solidly
grounded,
The
magnitude
of overhigh
voltages
due togrounded
ground faults
depends onsystems),
the method
system grounding
grounded,
resistance
grounded,
resistance
or ungrounded
theofequivalent
sequence(solidly
impedances
seen
resistance grounded, high resistance grounded or ungrounded systems), the equivalent sequence impedances seen
* Corresponding author. Tel.: +38513795427
* Corresponding
Tel.: +38513795427
E-mail address:author.
kosjenka.capuder@siemens.com
E-mail address: kosjenka.capuder@siemens.com
This work has been supported in part by the Croatian Science Foundation under the project number IP-2013-1118.
This work has been supported in part by the Croatian Science Foundation under the project number IP-2013-1118.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review
the organizing
committee
1877-7058 ©under
2017responsibility
The Authors. of
Published
by Elsevier
Ltd. of ICTRAM 2017.
Peer-review under responsibility of the organizing committee of ICTRAM 2017.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the organizing committee of ICTRAM 2017.
10.1016/j.proeng.2017.09.718
Kosjenka Capuder Bikić et al. / Procedia Engineering 202 (2017) 305–311
Kosjenka Capuder Bikić et al. / Procedia Engineering 00 (2017) 000–000
306
2
from the fault location and the fault impedance. Their duration depends on the fault clearance time and therefore on
the design of the protection system. An estimation of the duration and magnitude of these over voltages is crucial for
selection of surge arresters in most power systems. [1]
The grounding system determines the over voltages that can occur during a fault to ground. A single phase to
ground fault shifts the ground potential at the fault location, depending on the severity of this shift on the grounding
configuration. On a solidly grounded system with a good return path to the grounding source, the shift is usually
negligible. On an ungrounded system, a full offset may occur and the phase-to-ground voltage on the unfaulted
phases approaches the phase-to-phase voltage value. [2]
Nomenclature
HV
MV
LV
EFF
high voltage winding of a transformer
middle voltage winding of a transformer
low voltage winding
Earth fault factor
2. Calculation
Model for single-phase to earth short circuit on MV side is presented in Pogreška! Izvor reference nije
pronađen., where UrHV_U, UrHV_V and UrHV_W are rated voltages for each phase on HV side of the transformers with
corresponding network inductance ZNHV. UrMV_U, UrMV_V and UrMV_W are rated voltages for each phase on MV side
of the transformers with corresponding network inductance ZNMV.
The calculation of the parameters from Fig. 1 is presented in [3]. Two different cases were considered:
• MV network solidly grounded (ZNgr = 0 in Fig. 1)
• MV network ungrounded (ZNgr = ∞ in Fig. 1)
Each of those two cases will be analyzed for a range of transformer ground impedances 0 ≤ Zgr ≤ ∞.
Fig. 1 Model for single phase to earth fault on MV side
For short-circuit on MV side, new position of neutral point is calculated according to Fig. 1:
U NMV = I 7 ⋅ Z gr [ kV ]
(1)
Kosjenka Capuder Bikić et al. / Procedia Engineering 202 (2017) 305–311
Kosjenka Capuder Bikić et al. / Procedia Engineering 00 (2017) 000–000
307
3
The neutral point shift of MV system is capacitevely transffered to LV system (Fig. 2) as described in (2), where
CMV/LV is a capacitance between MV and LV winding and CLV0 and CMV0 are capacitances betwen LV (MV)
winding and earth.
Fig. 2 Capacitance system between MV and LV winding
U NLV = U NMV ⋅
CMV / LV
[ kV ]
CMV / LV + CLV 0
For a transformer with vector group Yyd, the voltage system of MV and LV winding is as presented in Fig. 3.
Fig. 3 Voltage system for MV and LV winding
Maximum voltage on LV winding ULVmax is calculated according to:
(2)
Kosjenka
Capuder
al. / Procedia
Engineering
202 000–000
(2017) 305–311
Kosjenka
Capuder
Bikić etBikić
al. / et
Procedia
Engineering
00 (2017)
308
4
2
U LV max = U NLV
2
U
U 
+  LV  − 2 ⋅U NLV ⋅ LV ⋅ cos α [ kV ]
3
 3 
(3)
3. Case study
Calculation of transient over voltages will be performed on a real case transformer. Input data for the
transformer is presented in Table 1, where Sr is rated power of the transformer; UrHV, UrMV and UrLV are rated
voltages; uscHV/MV, uscHV/LV and uscMV/LV are rated short-circuit voltages between windings referred to rated power Sr.
Table 1 Input data of the transformer
Sr
[MVA]
370
Vector
group
Yyd
UrHV
[kV]
220
UrMV
[kV]
123
UrLV
[kV]
24
uscHV/MV
[%]
16.1
uscHV/LV
[%]
44.77
uscMV/LV
[%]
25.16
SscHV
[MVA]
26700
SscMV
[MVA]
8700
3.1. Results
As mentioned, two different cases were considered:
• CASE 1: MV network solidly grounded (ZNgr = 0 in Fig. 1)
• CASE 2: MV network ungrounded (ZNgr = ∞ in Fig. 1)
Each of them was analyzed for a range of transformer ground impedances 0 ≤ Zgr ≤ ∞. The faulted phase is phase
C, while the other two phases A and B are 'healthy' phases.
Results obtained for CASE 1 are presented in Table 2, where UNMV is neutral shift of the MV system; UA and UB
are voltages on unfaulted phases of the MV system; UNLV is neutral shift of the LV system and ULVmax is the
maximum voltage of the LV system.
Table 2 Results for CASE 1
RuzMVtr [Ω]
UNMV [kV]
0
1
2
3
5
7
10
25
50
100
500
∞
0
14.12∟-118.7°
21.02∟-99.5°
23.859∟-88.8°
25.855∟-78.2°
26.495∟-73.2°
26.86∟-69.4°
27.15∟-63.8°
27.2∟-61.9°
27.2∟-60.9°
27.22∟-60.2°
-
UA [kV]
69.31∟-357.5°
67.04∟-0.8°
68.22∟-3.1°
69.57∟-3.9°
71.15∟-4.2°
71.92∟-4.1°
72.5∟-4.1°
73.31∟-3.8°
73.57∟-3.7°
73.69∟-3.6°
73.79∟-3.6°
73.82∟-3.6°
UB [kV]
69.31∟-122.5°
73.9∟-122.3°
75.65∟-120.5°
75.89∟-119.3°
75.56∟-118.1°
75.22∟-117.5°
74.9∟-117.2°
74.28∟-116.7°
74.05∟-116.5°
73.94∟-116.5°
73.84∟-116.4°
73.82∟-116.4°
UNLV [kV]
ULVmax [kV]
0
5.19
7.73
8.77
9.51
9.74
9.88
9.98
10.00
10.00
10.01
-
0
18.53
20.91
21.90
22.59
22.82
22.95
23.05
23.07
23.07
23.07
-
The voltage vector system for the faulted MV winding for ground impedance of 2Ω is presented in Fig. 4.
Kosjenka Capuder Bikić et al. / Procedia Engineering 202 (2017) 305–311
Kosjenka Capuder Bikić et al. / Procedia Engineering 00 (2017) 000–000
309
5
Fig. 4 Voltage vector system of MV winding for CASE 1
Vector uT (green in Fig. 4) is the neutral shift of the MV system which is then transferred to the LV system
according to (2) i (3) as shown in Fig. 3.
The neutral point shift of the faulted system for different transformer grounding impedances behaves as shown in
Fig. 5. Maximum neutral point shift occurs for the ungrounded voltage system and in this case it's 0.3 p.u. of the
rated phase-to-phase MV voltage.
Fig. 5 Neutral point shift for 0 ≤ Zgr ≤ ∞, CASE 1
Kosjenka Capuder Bikić et al. / Procedia Engineering 00 (2017) 000–000
Kosjenka Capuder Bikić et al. / Procedia Engineering 202 (2017) 305–311
6
310
Results obtained for CASE 2 is presented in Table 3.
Table 3 Results for CASE 2
RuzMVtr [Ω]
UNMV [kV]
0
1
2
3
5
7
10
25
50
100
500
∞
0
15.87∟-137.1°
29.60∟-125.4°
40.23∟-115.5°
53.50∟-101.1°
60.27∟-91.9°
65.09∟-83.6°
69.95∟-69.9°
70.75∟-65°
70.90∟-62.5°
71.00∟-60.5°
-
UA [kV]
103.9∟-23.7°
102∟-26.3°
102.4∟-28.9°
104.3∟-30.8°
108.9∟-32.5°
112.5∟-32.9°
115.8∟-32.7°
120.5∟-31.4°
121.9∟-30.8°
122.5∟-30.4°
122.9∟-30.1°
123∟-30°
UB [kV]
103.9∟-96.3°
107.8∟-98.1°
112.3∟-98.6°
116.2∟-98.4°
121∟-96.9 °
123.2∟-95.6°
124.3∟-94.2°
124.4∟-91.7°
123.8∟-90.8°
123.5∟-90.4°
123.1∟-90.1°
123∟-90°
UNLV [kV]
ULVmax [kV]
0
5.83
10.88
14.79
19.67
22.16
23.93
25.72
26.01
26.07
26.10
-
0
19.13
23.91
27.67
32.42
34.85
36.59
38.35
38.64
38.69
38.73
-
As in CASE 1, the neutral point shift of the faulted system behaves as shown in Fig. 6. Maximum neutral point
shift occurs for the ungrounded voltage system and in this case it's 1 p.u. of the rated phase-to-phase MV voltage.
Fig. 6 Neutral point shift for 0 ≤ Zgr ≤ ∞, CASE 2
Kosjenka Capuder Bikić et al. / Procedia Engineering 202 (2017) 305–311
Kosjenka Capuder Bikić et al. / Procedia Engineering 00 (2017) 000–000
311
7
4. Conclusion
In symmetrical three phase systems, in case of ground faults, resulting three phase voltages against earth are
different in magnitude and phase compared to their normal service values. These voltages are then transmitted to
physically adjacent but potentially unconnected systems due to capacitive coupling to affected earth - faulted
system, to earth and to each other. This causes a shift of neutral point in delta connection or of neutral point in star
connection. The resultant voltages against earth on the terminals of the shifted system can be higher than voltages in
the normal service on windings under consideration. Therefore, the calculation and analysis of such faults is
necessary in order to ensure the safety of the transformer.
In this paper, the case of a real transformer in service was analyzed to determine the influence of the grounding
configuration on both the transformer and the network system. The results naturally confirm that for a solidly
grounded transformer, there is no neutral point shift. However, for an ungrounded transformer, the neutral shift can
reach the phase-to-phase voltage in case of an ungrounded network system. For solidly grounded network system,
the neutral point shift is not as severe but it should be taken into account in the transformer design process.
References
[1] IEC 60071-2:1996-12 Standard, Insulation Co-ordination – Part 2: Application Guide
[2] Juan A. Martinez-Velasco, Universitat Politecnica de Catalunya, Barcelona, Spain, Transient Analysis of Power Systems; solution
techniques, tools and applications. IEEE Press
[3] K. Capuder Bikić, G. Plišić, Ž. Štih, Generic Approach to Calculation of Short Circuit Currents in Power Transformers, 3rd International
Colloquium Transformer Research and Asset Management, Split, Croatia, 2014